The disclosure relates generally to wind turbines and, more particularly, to improved bearing configurations for a wind turbine.
Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy from wind using known airfoil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
To ensure that wind power remains a viable energy source, efforts have been made to increase energy outputs by modifying the size and capacity of wind turbines. One such modification has been to increase the length of the rotor blades. However, as is generally understood, the loading on a rotor blade is a function of blade length, along with wind speed and turbine operating states. Thus, longer rotor blades may be subject to increased loading, particularly when a wind turbine is operating in high-speed wind conditions.
During the operation of a wind turbine, the loads acting on a rotor blade are transmitted through the blade and into the blade root. Thereafter, the loads are transmitted through a bearing, also referred to as a pitch bearing, disposed at the interface between the cantilevered rotor blade and the wind turbine hub. Typically, conventional pitch bearings include an inner ring, an outer ring, and two rows of balls, also referred to as rolling elements, concentrically disposed within separate raceways defined between inner and outer bearing races, with each rolling element being configured to contact its corresponding raceway at four separate contact points. This type of bearing is commonly referred to as a four-point bearing. In known bearing configurations, the predominant load applied to the bearing by the cantilevered blade is in the form of a moment that pries the bearing inner ring out of the outer ring. Typically, any bearing that is expected to do the job of a pitch bearing should have maximum capacity for moment rather than being designed to handle pure axial or radial load. Inside a bearing, this moment translates into forces on the rolling elements that act mainly parallel to the axis of the blade.
Unlike regular ball bearings, normal operation of pitch bearings in wind turbines involves oscillations about a set pitch angle as opposed to continuous rotation at high speed in one direction. Under ideal loading conditions, the loads transmitted through the pitch bearing are distributed evenly over all of the rolling elements. However, due to dynamic loading on the pitch bearing and the difference in stiffness between the hub and the rotor blade, only a percentage of the rolling elements actually carry the loads during operation of the wind turbine. As a result, the stresses within such load-carrying rolling elements tend to exceed the design tolerances for the pitch bearing, leading to damage and potential failure of the pitch bearing. Moreover, under dynamic loads, the rolling elements of conventional pitch bearings tend to run up and over the edges of the raceways, resulting in the rolling elements having reduced contact areas with the raceways. This leads to an additional increase in the stresses within the rolling elements, thereby further increasing the potential for damage to the pitch bearing components. In addition, the large bending and shear forces created result in unwanted deflections of the bearing races (potato chip). Similar issues may be present in conventional yaw bearings for wind turbines.
Thus, it is highly desirable to provide a wind turbine blade bearing configuration that addresses one or more of the issues described above.